A Wideband Power Efficient Inductive Link for Implantable Biomedical Devices using Multiple Carrier Frequencies

Abstract

This thesis presents a novel inductive link for wireless transmission of power and data to biomedical implantable microelectronic devices using multiple carrier frequencies. Achieving higher data bandwidth without compromising the power efficiency is the driving force to use two separate carriers. Two separate pairs of coils have been utilized for inductive power and forward data transmission. One major challenge, however, is to minimize the interference among these carriers especially on the implantable side, where size and power are highly limited. Planar power coils with spiral shape are optimized in geometry to provide maximum coupling coefficient, k. The data coils are designed rectangular in shape and wound across the power coils diameter to be oriented perpendicular to the power coil planes. The goal is to maximize data coils direct coupling, while minimize their cross-coupling with the power coils. The effects of coils geometry, relative distance, and misalignments on the coupling coefficients have been modeled and experimentally measured.

A prototype system is built to test the functionality of the approach. Frequency Shift Keying (FSK) is used to transmit data using two carriers at 6 and 12 MHz. Power is transmitted at 500 kHz. We achieved power efficiency of 43% at a nominal distance of 10 mm. We also achieved very minimal power interference on to data using the devised coil arrangement. Initial results show the functionality of the design. The inductive link needs more tuning to reduce the Bit Error Rate (BER) for successful operation of an implantable micro stimulating system.

In addition a novel back telemetry link is developed to transmit data back from the implant. This data gives information related to received power, electrode impedances etc, to the external part of the system operation and helps achieve a more stable closed loop system. We have also designed and developed a Manchester encoded FSK Data demodulator circuit to be used in our multi carrier system but have not tested it.